Over the past several years, a significant increase in recording density in thin-film media has been achieved, and there is a continuing effort to increase recording density further.
A number of magnetic properties in a thin-film media are important to achieving high recording density. One of these is coercivity, defined as the magnetic field required to reduce the remanence magnetic flux to zero, i.e., the field required to erase a stored bit of information. Higher coercivity in a medium favors higher storage density by allowing adjacent recorded bits to be placed more closely together without mutual cancellation. Typically, coercivity values of greater than about 1,200 Oe (Oersted) are compatible with high recording density.
Another property of a thin-film medium which is relevant to recording density is bit shift or peak shift. This phenomenon is related to the broadening of signal peaks, as well as to the intersymbol interference. To the extent that the bit shifting limits the resolution at which adjacent voltage peaks can be read, it places an upper limit on recording density. That is, the higher the bit shift values in a thin-film medium, the lower the recording density which can be achieved.
Flying height, i.e., the distance which a read/write head floats above the spinning disc, is another important factor in achieving high recording density. It can be appreciated that less overlap of voltage signals in adjacent magnetic domains in the disc occurs as the read/write head is moved closer to the disc surface, allowing recording density to be increased. The flying height is limited principally by surface irregularities on the disc.
Thin-film media having high coercivity and reasonably low bit shift values have been prepared using aluminum substrates. Typically, the aluminum substrate is first plated with a selected alloy plating, such as a nickel/phosphorus plating, to achieve a requisite surface hardness, then polished to remove surface nodules which form during the plating process. Because the nodules have varying degrees of hardness, the polishing step tends to leave surface irregularities in the form of surface depressions or mounds.
After surface preparation, the metal substrate is moved through a sputtering apparatus, where successive sputtering steps are used to deposit an underlayer and a cobalt-based thin-film magnetic layer. The underlayer is required for forming a crystalline surface which effectively orients the c-axis of the magnetic film crystals either in-plane for longitudinal recording, or out-of-plane for vertical recording. A carbon coating is applied over the magnetic layer for lubricating and wear-resistance properties.
This method for producing a metal-disc thin-film medium is illustrated in co-owned U.S. Pat. No. 4,816,127. Here a chromium underlayer is applied to a coated metal substrate by sputtering to a final underlayer thickness of 1,000-4,000 .ANG.. It is necessary, in forming an underlayer with the desired crystal anisotropy, to perform the sputtering at an elevated temperature, typically above about 200.degree.-300.degree. C. After the underlayer is formed, the disc is transferred to a second sputtering station, where a cobalt-based magnetic layer is sputtered onto the underlayer. The resultant disc can have a coercivity, with respect to longitudinal recording, of greater than 1,200 Oe.
Despite the favorable magnetic properties which can be achieved in a metal-disc thin-film disc of the type just described, the recording density of the disc is limited in flying height by irregularities on the surface of the disc (due to surface irregularities in the metal substrate surface). The best flying head distances which have been achieved with metal-substrate discs is about 6 .mu.inches.
It is possible to reduce flying height, and therefore to increase recording density, by forming a thin-film magnetic layer on a smooth-surfaced substrate, such as a glass or ceramic substrate. Thin-film media having glass or ceramic, or temperature-resistant polymer substrates have been proposed. However, difficulties in achieving performance characteristics needed for high recording density have limited this approach to date. Experiments conducted in support of the present invention, for example, indicate that thin-film media formed by prior art sputtering methods tend either to have relatively high bit shift values, e.g., greater than 18-20 ns, or relatively low coercivity values, e.g., less than 1,200 Oe. As discussed above, either low coercivity or high bit shift would limit the recording density which could be achieved in the disc.